Older people with mild cognitive impairment, especially when characterized by episodic memory loss, are at increased risk for dementia due to Alzheimer’s disease. Now a study by researchers from MIT, Cornell, and Massachusetts General Hospital has identified a key deficit unrelated to memory that may help reveal the condition early—when any available treatments are likely to be most effective.

The issue has to do with a subtle aspect of language processing: people with amnestic mild cognitive impairment (aMCI) struggle with certain ambiguous sentences in which pronouns could refer to people not referenced in the sentences themselves.For instance, in “The electrician fixed the light switch when he visited the tenant,” it is not clear without context whether “he” refers to the electrician or some other visitor. But in “He visited the tenant when the electrician repaired the light switch,” “he” and “the electrician” cannot be the same person. And in “The babysitter emptied the bottle and prepared the formula,” there is no reference to a person beyond the sentence.

The researchers found that people with aMCI performed significantly worse than others at producing sentences of the first type. “It’s not that aMCI individuals have lost the ability to process syntax or put complex sentences together, or lost words; it’s that they’re showing a deficit when the mind has to figure out whether to stay in the sentence or go outside it to figure out who we’re talking about,” explains coauthor Barbara Lust, a professor emerita at Cornell and a research affiliate at MIT. 

“While our aMCI participants have memory deficits, this does not explain their language deficits,” adds MIT linguistics scholar Suzanne Flynn, another coauthor. The findings could steer neuroscience studies on dementia toward brain regions that process language. “The more precise we can become about the neuronal locus of deterioration,” she says, “that’s going to make a big difference in terms of developing treatment.”